Medical ultrasonic imaging usually utilizes reflection of ultrasound at the interface of body tissues, receives and processes the echo containing characteristic information of the boy tissue to obtain a visible ultrasound image thereof. However, it may difficult to view small blood vessels in color Doppler imaging due to reverberation, restriction of the resolution, low vascular flow or low velocity of the vascular flow in the small blood vessels, sometimes the mall blood vessels are blurring or even cannot be found in the ultrasound image. Ultrasonic contrast imaging is now utilizes to make the blood flow imaging sharper by using contrast agent to enhance the back-scattering, which can help practitioners identifying and diagnosing particular diseases. Generally, contrast imaging micro-bubbles in the small blood vessel can reflect the structure thereof. However, if the diameter of the small blood vessel is close to the contrast imaging micro-bubbles, there will be few micro-bubbles in the small vessel, and that can result in unstable contrast imaging and the micro-bubbles are likely identified as random speckles in the image.
Conventional ultrasonic imaging includes the steps of converting electrical signals into ultrasound through a probe, transmitting the ultrasound to the target object, receiving the reflected and scattered echo beams from the target object, the echo beams then can be converted to electrical signals. A DSC (Digital Scan Conversion) and image post processing step can be performed after the electrical signal being signal processed, and the images obtained by the processing steps can be displayed or stored at last. In an ultrasonic imaging system utilizing contrast agents, an imaging processing unit is usually introduced before or after the DSC. For example, the imaging processing unit can utilize a MIP (Maximum Intensity Projection) method to reflect micro vascular morphology by accumulating flow paths of the contrast micro-bubbles in the small blood vessel. The principle of the MIP imaging can be described as: acquiring a plurality of frames of contrast image, and a pixel with maximum brightness (intensity) among the spatial corresponding pixels in each frame of image can be projected to a fixed template image. After the MIP processing, the contrast images of the micro-bubbles in multi-frame contrast image can be connected in the fixed template image, and the detected structure of the small blood vessels is finally outputted.
MIP operations may described according to the following equation (1):outputPixel(x,y,n)=max(inputPixel(x,y,1), inputPixel(x,y,2), . . . , inputPixel(x,y,n)) =max(inputPixel(x,y,n), outputPixel(x,y,n−1))  (1)wherein, inputPixel (x, y, 1) represents the actual luminance of a pixel with coordinates (x, y) of the first frame input image, inputPixel (x, y, 2) represents the actual luminance of a pixel with coordinates (x, y) of the second frame input image, . . . , and so on, thus inputPixel (x, y, n) represents the actual luminance of a pixel with coordinates (x, y) of the nth frame input image, and 1,2, . . . N represents time sequence; similarly, outputPixel (x, y, n−1) represents the displayed luminance of a pixel with coordinates (x, y) of the n−1th frame output image, and outputPixel (x, y, n) represents the displayed luminance of a pixel with coordinates (x, y) of the nth frame output image.
The popular commercial ultrasonic imaging equipment usually have the function of continuous contrast imaging, although the names are different but the imaging principles are all based on the MIP imaging method. Those conventional MIP methods are referred to as conventional MIP in this disclosure. Please refer to FIG. 1, the upper row of rectangular frames represent an original ultrasonic contrast image sequence, the lower row of rectangular frames represent a MIP result image sequence using the conventional MIP. The input image at the MIP processing start point (shown as the “MIP function starts”) can be selected as the fixed MIP projection template, and the MIP projection period is shown as inverted brackets, each of the brackets denotes a projection period. Accordingly, the projection periods increase gradually in the conventional MIP, from the starting point of the MIP process to the ending point (shown as the “MIP function ends”), and finally, MIP projection period is equal to the MIP imaging period. MIP process starting point and ending point are usually system default or user-selected. That is, the maximum luminance value of all selected image sequence is projected onto the first frame image (the template image).
Conventional MIP imaging techniques are capable of providing spatial information of existence lesions or regions of interest, such as morphology of the small blood vessels, but meanwhile part of the time information are lost, such as fade time of contrast agents. The time information of the suspected lesion, such as fast-forward and fast-rewind, or fast-forward but slow rewind, can help doctors diagnosing whether the tumor is benign or malignant. On the other hand, the conventional MIP imaging technology is a contrast image accumulation and overlay technology, the longer the projection period, the larger the cumulative error, such that the contrast of the vessels and tissue decline, and the MIP imaging result becomes worse.